Formaldehyde is a known carcinogen and allergen. For this reason, the Occupational Safety and Health Administration (OSHA) in the United States has set an eight-hour exposure limit of 0.75 parts per million and a 15-minute exposure limit of 2 parts per million for formaldehyde vapor. Despite its toxicity, formaldehyde is a high volume industrial compound. It is used, for example, to prepare polymeric materials that find widespread use in various building materials including foam insulation, particle board, carpet, paint, and varnish. Out-gassing of residual formaldehyde from these building materials makes formaldehyde one of the most prevalent indoor air pollutants. Formaldehyde is also a by-product of the combustion of organic materials. Thus, formaldehyde is a common outdoor pollutant as well arising from automobile exhaust, methane combustion, forest fires, and cigarette smoke.
While in North America strict formaldehyde out-gassing limitations are placed on building materials, this is not the case in all parts of the world. In some Asian countries, for example, few restrictions are placed on building materials. Combined with an increased use of biofuels to heat homes and to run automobiles, dangerous levels of formaldehyde vapor may occur in both indoor and outdoor air. For this reason, there is an immediate need for solutions to mitigate human exposure to formaldehyde vapor both as an indoor and an outdoor air pollutant.
The high volatility of formaldehyde (it is a gas at room temperature) makes it extremely difficult to capture by the mechanism of physisorption alone. However, because formaldehyde is reactive, it can be more readily captured through chemisorption. With chemisorption, the formaldehyde vapors are captured by chemically reacting with the sorbent itself or with chemicals impregnated in the sorbent. Thus, the key to making high capacity sorbents for formaldehyde is to provide a sorbent with many reactive sites for formaldehyde.
One typical sorbent material that has been used for capturing formaldehyde is based on activated carbon scaffolds. The scaffold of activated carbon, however, is relatively inactive and this inactivity makes it difficult to incorporate a high density of reactive groups into the activated carbon scaffold itself For this reason, most of the efforts in making sorbents for formaldehyde have been focused on finding impregnation chemistries that can react with formaldehyde. Thus, the activated carbon scaffolds are typically impregnated with various chemistries that react with formaldehyde. The two most common impregnation chemistries used for formaldehyde capture are the sodium salt of sulfamic acid and ethylene urea co-impregnated with phosphoric acid. Various other metal salts have also been used.
Impregnation in general has some drawbacks for making sorbents. First, impregnation chemistries can migrate and this is problematic especially if other sorbents are used in the same product. Another disadvantage to impregnation is that it removes activated carbon's capacity for adsorbing volatile organic compounds (VOCs). The impregnation chemistry occupies the pores of the activated carbon thus reducing the surface area available to capture non-reactive vapors that are captured by physisorption only.